California seed zone boundaries were first proposed in 1946, revised, and published as a joint report between the U.S. Forest Service and CAL FIRE in 1970. The 85 seed zones are defined by major areas in California having similar climatic, topographic, and soil conditions. These are areas where plant materials can be moved or transferred with minimal risk of being poorly adapted to a new location. Each seed zone is labelled by a three-digit identifier that reflects the layer’s creation. The first digit indicates the physiographic and climatic region. The second digit gives the subregion, which captures the next lower level of environmental changes known to affect growth and adaptability of plants. The last digit denotes the subzone. Subzones are limited to about 50 miles in latitude and further refine the uniformity of environment within each subregion. Where possible, boundaries follow natural or physical features. For more information, see the original publication: https://www.fs.usda.gov/research/treesearch/41438.
